55 



MACTRA. 



(Plate VI. fig. 2.) 



Shell inequilateral, equivalve. Hinge having the 

 primary tooth complicated, with an adjacent sinus, 

 lateral ones remote and mutually inserted, (fig. 1.) 

 Shape various, sub-triangular, or broader than 

 long, sometimes gaping. The form of the hinge 

 is very remarkable. The compound middle tooth 

 and the adjoining hollow, filled with an elastic car- 

 tilage connecting the two valves, are easily to be 

 distinguished, when their construction has once 

 been attentively observed. 



We may here remark, that whenever an internal 

 cartilage does occur, it is generally of a triangular 

 shape, or of one tending to a three-sided prism, the 

 acute angle of which is placed under the apex of 

 the shell. It is not possible to conceive a con- 

 formation more beautifully adapted to the motion 

 of the valves and the articulations of the hinge. 

 This cartilage, it appears, should be considered as 

 performing some other functions than those of the 



